[cas3]

First thing I would do is soak them in a barrel of molasses. Its really cheap, lasts all summer (does not wear out, like evaporust, which is really good but very expensive), you buy a bag at the farm store, its horse feed.

Dump a 25lb bag in a 55 gallon drum, or kids pool full of water and soak them for weeks.
This will be only the first step, get the surface rust off and certainly other more drastic methods will be required to get things loose. Again, its cheap and lasts all summer.

Folks in a mild climate should always have a batch to dunk old parts into out behind the shop.